ABSTRACT

Field experiments are conducted to determine wave energy transmission characteristics of San Pedro Breakwater, Los Angeles, California. Sea surface elevations are measured by means of Vibration Pres sure Transducers placed on the sea floor approximately 16 meters from the sea shake. The wave sensors are placed at three locations with two sensors at each location. Two of the sensor sites are on the Oceanside of the breakwater; one of them is on the harbor side. Signals from the sensors are sent to a frequency division multiplexer located on top of the breakwater and then transmitted through an undersea cable (approximately 2 miles long) to the shore recording station at which the signals are separated, digitized, and recorded on a nine track digital tape recorder compatible with IBM 360 computer.

Spectral analysis of the taped data is undertaken. The results show that the predominant wave energy centers on wave with period equal to 16 seconds. It also shows that approximately 50% of the incident wave amplitude is reflected by the breakwater and returns to the Oceanside. For this wave period, the wave transmission coefficient is about 30% with the remainder of the wave energy dissipated within the breakwater.

INTRODUCTION

Ocean waves impinging upon a rubble mound breakwater may be partially reflected by the breakwater, may be attenuated by viscous dissipation and turbulence while passing through the porous spaces between rocks, with the remainder transmitted to the shoreward side of the structure. Engineers are very much interested in the transmission characteristics of the breakwater because the very purpose of a breakwater is to cause reduction of wave heights in the lee side.

Although a relatively large number of laboratory studies have been conducted to determine the reflection and transmission characteristics of various breakwater models, only a very few number of experiments are conducted in a prototype situation. Recently, Thornton and Calhoun (1972) conducted a prototype study at Monterey Harbor Breakwater, Monterey, California. The transmission characteristic of the breakwater reported by their study shows larger energy transmission occurs at the lower frequencies. They showed that wave transmission coefficients (transmitted wave height divided by incident wave height) are in excess of 40% for wave periods of 25 seconds or longer. Such results further reinforce -the fact that the rubble mound breakwater is very much subject to wave penetration especially with waves of longer periods.

The present work is part of a stucrycur rently being conducted at the University of Southern California on the harbor surging prob lem with wave energy entering through both the harbor entrance and the permeable breakwater. The wave transmission characteristics of the breakwater obtained through field experiments is intended for providing information to be incorporated in the general theory developed for the harbor surging Problem. The field experiment has not been completed, thus the world reported herein represents the initial stage of the study with emphasis on the instrumentation system with SITEL initial results.

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